1. Field of the Invention
Aspects of the present invention relate to an electrode assembly and a secondary disposing a ceramic separator between a positive electrode plate and a negative electrode plate, and a secondary battery with the same.
2. Description of the Related Art
As opposed to a primary battery, a secondary battery is rechargeable, and thus, is reusable. Secondary batteries are generally used as the main power source of portable devices for communication, information processing, and audio/video applications. Currently, interest in secondary batteries is increasing and improved secondary batteries are rapidly being developed. For example, secondary batteries that are super-light, have a high energy density, are environment-friendly, have a high output voltage, have a low electric discharge rate, and have a longer lifespan, are being developed.
Secondary batteries are classified into Ni-MH batteries, Li-ion batteries, and the like, based on electrode active materials. Li-ion batteries may be classified based on the type of electrolyte, for example, a liquid electrolyte, a solid electrolyte, or a gel-type electrolyte. Secondary batteries are classified into various types, such as, a can type, a pouch type, and the like, based on the shape of a can within which the electrode assembly is accommodated.
The weight-to-energy density of a Li-ion battery is much greater than that of a primary battery, and thus, can be manufactured as a super-light battery. The average voltage of each cell of a Li-ion battery is about 3.6V, and each cell is three times more compact, in comparison with the average voltage of about 1.2V, of other secondary batteries, such as Ni—Cd batteries and Ni-MH batteries. Lithium ion batteries have an electric discharge rate of less than 5% a month, at a temperature of about 20° C., which is about one third of the electric discharge rate of a Ni—Cd battery or a Ni-MH battery. A lithium ion battery does not use heavy metals, such as, cadmium (Cd), or mercury (Hg), and thus, is environment-friendly. A lithium ion battery can be recharged more than one thousand times in a normal state. Accordingly, research into secondary batteries is being rapidly performed, based on the above-described advantages.
A conventional second battery forms a bare cell by disposing an electrode assembly, which is formed of a positive electrode plate, a negative electrode plate, and a separator, within a can formed of either aluminum or aluminum alloy. An opening formed in the can is sealed with a cap assembly. Electrolyte is injected into the can, via the opening. When the can is formed of either aluminum or an aluminum alloy, the battery can be light, due to the lightness of aluminum. When the battery is used for a long time under a high voltage, the battery does not rust.
The sealed bare cell is connected to safety devices, such as, a positive temperature coefficient device (PTC), a thermal fuse, a protective circuit module (PCM), and/or other battery accessories. In this state, the bare cell is contained within a separate hard package, or is contained within a molding formed using a hot melt resin.
The separator of the electrode assembly is disposed between the positive electrode and the negative electrode, in order to prevent a short-circuit therebetween. However, when the separator does not have a sufficient permeability and/or wettability with respect to the electrolyte, the separator restricts the movement of lithium ions between the positive electrode and the negative electrode, and thereby results in degraded electrical properties.
The separator prevents overheating of the secondary battery, by acting as a safety device. However, when the temperature of the battery suddenly increases due to, for example, thermal transmission from outside, and the like, the temperature of the secondary battery may increase, even though minute through-holes of the separator are closed. Consequently, the separator may be damaged.
A second battery generally has a high capacity. Accordingly, when large amount of current flows in the secondary battery in a comparatively short time, the temperature of the secondary battery is not decreased by stopping the current flow, even though the minute holes of the separator are closed. On the contrary, due to the generated heat, the separator may become fused. Consequently, an internal short-circuit may occur, due to damage to the separator.
There is a need to prevent the internal short-circuit between electrodes, even at a high temperature. Accordingly, the separator is a ceramic separator with a porous film. In this instance, the ceramic separator is formed by combining particles of a ceramic filler with a heat-resistant binder.
In the conventional art, the ceramic separator generally uses alumina (Al2O3) particles having the same average particle size to form the ceramic filler. Since the alumina (Al2O3) particles have the same particle diameter, a pore compaction between the particles is reduced, and an insulating property of the ceramic separator is also reduced, as the thickness thereof increases.